Host their app at any major PAAS providers, such as EngineYard or Heroku. Check their code into a private Github repository that you can assign them as the owner. That way they have access to the source code and can create a new instance quickly using the repository as the source.
I don't see the need to create an entire service mirror for a Rails app, unless there are specific configuration needs that can't be contained in the project or handled through capistrano.